cAMP and cGMP have been implicated in the control of a wide range of metabolic functions. A partial list of processes believed under their control includes glycogenolysis, lipolysis, secretion, ion transport, egg maturation, contractility, cell shape, and terminal differentiation. In carrying out many of their roles, these nucleotides are mediating the action of hormones. Alterations in intracellular levels of cAMP and cGMP have also been associated with malignant transformation in a number of cell lines. Hence, understanding how the levels of these nucleotides are regulated should provide insight into the mechanism of a host of physiological functions, and also into the biochemical basis of malignancy. The enzymes synthesizing these nucleotides, adenylate cyclase and guanylate cyclase, are believed to be primary control sites for regulation of the cyclic nucleotide levels. In vitro studies of the enzymes have uncovered a myriad of possible regulatory factors, and others certainly remain to be discovered. It has been difficult, however, to determine how, or even if, these factors affect the enzymes actvity in vivo. It is the crux of this proposal that the complex task of deciphering the regulatory interactions of these enzymes would be greatly facilitated by focusing on a relatively simple organism that holds forth the promise of a genetic as well as biochemical dissection. Specifically, the cellular slime mold Dictyostelium discoideum is presented as an organism meriting close examination as a model system for the study of cyclic nucleotide synthesis. We plan first to conduct a detailed enzymatic analysis of the D. discoideum adenylate and guanylate cyclases, and then to utilize the knowledge obtained to analyze mutants defective in their cyclic mechanism.